What makes a hosta go dormant?
Note, we are fortunate that one of the top, if not the top, shade plant botanists in the world, W. George Schmid, has allowed me to print this helpful article.
About Flowering and Dormancy — By W.G. Schmid (© 2008 )
Hostas have a mind of their own when it comes to flowering and dormancy. Given a steady climate and weather, they flower faithfully and go dormant just at the right time. If the weather turns on them, like our weather during the last few years, our usually reliable hostas become finicky and may not flower at all, and worse yet, go dormant during the middle of the summer. They just will not “do their thing” if the environ-ment turns on them. First let’s concern ourselves with in-ground plants only. Observing them “doing their own thing” gave me the idea of recording some of their now unusual behavior and compare it with their normal behavior when it comes to flowering and going dormant. Most of all I have to say applies to and has been determined for species. Sports and closely related cultivars may behave the same way, but most other cultivars (hybrids) have a mixture of genes and my determinations may not fully apply to them or apply in a modified way. Pots are an artificial habitat and can be moved around to change conditions. This, of course, changes the environment on a whim and my notes do not apply to potted hostas. Notwithstanding, I will deal with them peripherally. Here is what I found, stated simply:
1) Flowering periodicity (timing) is rooted in genetics.
2) Dormancy is triggered by environment.
Genetics dictate if a plant is early-, mid-, or late-flowering. For example, H. kikutii is late flowering. H. montana ‘Aureomarginata’ comes from a southern species of H. montana population so it comes up early wherever it grows. It gets zapped north or south if there is a late freeze. This trait cannot be changed by environ-mental factors because it is an evolutionary adaptation to the original habitat climate. If you don’t believe it, just observe H. ‘On Stage’, also a H. montana sport, but one that originated with a more northern population and comes up late enough to miss all the late freezes. So you need to know not only what species is involved, but also what latitude the parent population of a given sport grows. Thus, for species growing all over Japan, like H. montana, the habitat location is imperative. Hostas blooming time is set by genetic imprint, regardless of environment. Yet, if a certain species has habitats that spread over different latitudes, there will be a difference in blooming time, due to an evolutionary adjustment to the location. Most hosta species do not spread far and wide, so the blooming time is usually a uniform time. Here in Tucker, Georgia, H. kikutii blooms mid-October until about November and I have some still blooming (in the final stages) out there now. There is little or no difference in the blooming time between species on top of Black Rock Mountain (3600 feet) at the cabin or in our home garden in Tucker (1200 feet). So the primary timing for blooming start has nothing to do with elevation or temperatures (well, a little bit, but let’s not get complicated!). Cultivars are another matter they have convoluted and mixed genes and it is impossible to draw conclusions since we don’t have their true genetic makeup in most cases. They may be a hybrid between an early and late bloomer and one or the other might be dominant. This could only be found out by observation and recording the results for each cultivar.
The rising of hostas in early spring is triggered by daylength (the days beginning to lengthen), the sun rising higher in the sky, and the outside temperatures beginning to rise. These environmental factors trigger a change in the supply of plant growth regulators (mostly hormones). The concentration of each growth regulator changes in response to the favorable growing conditions; as a result, plant growth is stimulated. But the internal growing mechanism is also controlled by the genetic make-up, meaning that the exact required values of warming and day length required differ depending on the original, evolutionary habitat. That is why toad lilies wait until fall to bloom and daffodils come up while it is still snowing.
Shorter day lengths, i.e., the reduction in sunlight stimulates dormancy but that is INDEPENDENT of the flowering trigger. Late blooming hostas bloom whenever they bloom in Japan (species only). Thus, onset of dormancy can happen while there are full blooms on the raceme. In fact, dormancy can be forced by early freezes while the hostas are in full bloom due to the fact they are late bloomers. So dormancy is environmental and blooming is genetic. Hopefully they don’t overlap as they do with late blooming hostas sometimes. H. ‘Tardiflora’ has inherited the late-blooming habit from H. sparsa. Here it blooms in mid-fall (a bit later than H. kikutii).
It is difficult to fool plants, including hostas. Experimenting around with daylength or surrounding temperatures may be fun, but usually is prone to fail. Yes, you can cover the plant with blankets and change daylength but you will never be able to copy the natural rhythm of daylength. Likewise, it is impossible to fake air temperatures (which are part of the dormancy trigger), unless the plants are grown in an isolation chamber or a greenhouse. Root and rhizome chilling can be done in a pot, but for inground plants it makes no sense since the cold will not penetrate the ground deep enough if you just put a pile of ice cubes on top of the plant. Potting and freezing the plants in a freezer has been tried by people who want to have hostas up and showing in February Flower Shows. It works, but it is difficult to time, since the warm-up timing has to be adjusted to early- or late-blooming varieties (genes). Anyway, soil temperatures do not matter for dormancy, because the plants will already be dormant when the soil finally freezes solid up north. In the south our soil never freezes (or maybe an inch or so). Cold periods have nothing or little to do with size. That is primarly controlled by genes, and the amount of moisture and nutrient flow. Hostas add sugars to the rhizome for next years growth after expending a bunch for blooming and setting seed (late August to early November – location dependent). If the late summer/early fall is dry, they cannot make enough sugars to add size. During drought periods, the rhizome even diminishes in size and as we have seen here in the South, making the rhizome dwindle away and the plant simply disappears after two or three drought years as the rhizome gets smaller and smaller.
There is some connection to length of growing season and plant size. In the North, the season is shorter and the plants must grow faster to complete their seasonal cycle. The leaves will also grow faster and larger. In the South they can linger longer and the leaves grow slower and smaller (and maybe more numerous) because they have more time to complete their cycle. Blooming is still genetically time, but plant growth depends on many environmental factors. Some very late bloomers will be sometimes be caught by early freezes and the flowers will never open or even develop because early freezes destroy the blooms and trigger plant dormancy. The same happens during severe drought. A lack of moisture and/or high heat will make the plant reduce its transpiration load by shedding leaves or going dormant altogether to save what is left of the rhizome.
How far south will hostas grow? Zone 10 is NOT a hosta habitat. Yes, you can try with a few H. plantaginea derivatives and perhaps store them in a cool area during winter. I have seen them in sub-tropical Italy in pots. Being in pots exposes the entire plant to cold air temperatures and they may get enough dormancy to return in spring. They fail in the ground, though, because the ground stays warm all season. Whatever might let them succeed in zone 10 would certainly not be a garden habitat, but an artificial one, like that of a hot house for growing orchid in the North. Some the methods you suggest may work, but all are also artificial methods. If you had a million, you could build a cool house with the temperatures adjusted to zone 6 and lots of artificial watering during late summer. That might work but it will cost a fortune. Here in the South, people grow some hostas in zone 8, but that is about as far south as I have observed them (in Savannah). I have never seen hostas successfully grown in the ground further south in Florida (zones 9 and warmer), but I have heard of a guy in Florida who installed an underground chilling system to keep his hosta roots cold. It might be better to grow sub-tropical plants or cacti.